As a rule, vacuum lifters are used as crane accessories; however they are also mounted on conveyance means such as fork lifts with lift masts, on balancing equipment, manipulators, roller reversers or the like. Multiple mountings also are possible. Such vacuum lifters for instance allow raising and conveying rolls of wound bands, for instance made of paper, metal, plastic or laminates, and, depending upon the shape of the suction head, by coming to rest against the end face or the periphery of the prone or upright roll. Flat bodies also, for instance panels, may be transferred by such vacuum lifters.
A vacuum lifter of this species illustratively is described in the German Gebrauchsmuster 84 35 161.6. Its suction head is a plane suction plate to the lower side of which are mounted several concentrically spaced annular seals. These annular seals enclose mutually separated vacuum spaces which, when the suction plate is deposited on the pertinent adhesion surface of a load to be transported, will form sealed annular chambers when and if the load is covered. The "annular" seals need not be circular but may be of any other shape, for instance being oval or polygonal provided they form a closed ring. Nor is it necessary that they be one inside another. They may also enclose adjacent vacuum spaces or be distributed over several suction plates.
The vacuum spaces are connected through apertures in the suction plate with a vacuum source, for instance a pump. A valve is associated with each of the vacuum chambers outside the inner one, and is present in the connection to the vacuum source. Each valve is coupled to a sensor measuring the magnitude of the adhesion surface of the load when the vacuum lifter is lowered. The sensor is always mounted in such a way that it opens the valve when the adhesion surface is so big that the associated vacuum chamber shall be completely sealed upon deposition on the adhesion surface. In this manner only those vacuum spaces that form closed annular chambers after deposition of the vacuum lifter on the load shall reliably communicate with the vacuum source and therefore shall not suck-in unwanted air.
In particular, mechanical pickups are applicable as sensors to control the valves, and these sensors project downwardly beyond the plane of the annular seals and are forced upwardly by the load adhesion surface when suction is applied. However, other sensors, for instance optical ones, also are usable. The mechanical pickups always are mounted on the outside of the pertinent annular seal because thereby the associated valve shall reliably be opened only when the load adhesion surface is larger than the associated annular or vacuum chamber.
Now it may happen that the diameter of the adhesion surface of the load is precisely the same size as the outside diameter of one of the annular seals, so that the adhesion surface does not project outwardly beyond the annular seal. Consequently the associated sensor mounted on the outside of this annular seal will not measure, and in the event of being a measuring pin, will not be forced upward. In that event the associated valve shall not be opened, whereby, in turn, the vacuum space enclosed by this annular seal remains unconnected to the vacuum source and therefore is not evacuated. As long as the vacuum spaces inward from this vacuum space generate enough holding force to move the load, no drawback regarding moving the load is incurred for the time being.
However problems do arise when thereafter raising the vacuum lifter off the deposited load, particularly because it has been necessary to approximate when the vacuum chamber(s) was at neutral or because a time of more than sufficient duration was allowed to elapse in which the neutral was to be obtained. In this procedure the annular seals that were elastically compressed by means of the vacuum during the transport elastically move apart, so that during a short segment of the raising motion they remain in contact with the load adhesion surface. The inner, vented vacuum spaces can then draw in air, but not the outer vacuum space that was not subjected to vacuum during said transport. Because of the elastic outward motion of the annular seals, an undesired vacuum suction force has been created. If it is less than the weight of the load that was just deposited, then, as the vacuum lifter is raised off, detachment takes place impulsively with possible errors in control sequence. If the vacuum force were to be larger than the weight of the deposited load, then initially this load is carried along. The air which no longer is evacuated and now follows the load is the predominant cause of the collapse of the vacuum suction in this vacuum chamber, whereby the load crashes in uncontrollable manner and is damaged. Moreover, there is significant danger of an accident.
U.S. Pat. No. 3,999,795 describes a vacuum lifter of which the vacuum head comprises three suction plates, each with one annular seal. These three suction plates are not associated each with one valve, instead only with one central valve by means of which communication with the vacuum source may be set up. Moreover, compressed air equipment is provided, which includes an injector feeding a de-icing liquid. By switching the central valve, de-icing liquid can be periodically supplied to the vacuum chambers, the compressed air ensuring atomization.
Moreover, a vacuum lifter is disclosed in U.S. Pat. No. 4,865,420 which comprises a vacuum head with a plurality of suction cups, whereby for instance eggs can be moved. The vacuum head communicates with a vacuum pump acting as the vacuum source, and the vacuum head alternatingly can be connected to the suction or the pressure side of the vacuum pump. However such a design is unsuitable for vacuum lifters of the above species.